According to what i have learned, "Lexical" comes from the word Lexicography.
Lexico = word, graphy = picture.
Lexicography is the collection of words and pictures, serving as dictionary maker. It is a subfield extanded from applied linguistics (AL). If you happen to take teaching training course, you must aware of AL.

Lexical field or semantic field is the way of organizing related words and expressions into a system which shows their relationship to one another. For example, father, mother, uncle, and aunt,... belong to one lexical field.
Fathy